Electrolytic capacitors using metal having a valve action such as tantalum or aluminum are widely used in general since a large capacity can be obtained with a small size by forming a valve-action metal as an opposite electrode on an anode side into a shape of a sintered body, an etching foil or the like so as to enlarge a surface of a dielectric. Particularly, solid electrolytic capacitors using a solid electrolyte as its electrolyte is indispensable for size reduction, functional improvement and cost reduction of electronic equipment since they have properties such as ease to be made into a chip, suitability for surface mounting and the like in addition to the properties such as a small size, large-capacity and low-equivalent series resistance.
For small-sized large-capacity applications, this type of solid electrolytic capacitors generally has a structure in which a capacitor element is formed by winding an anode foil and a cathode foil composed of a valve-action metal such as aluminum with a separator interposed therebetween, a solid electrolyte layer is formed in this capacitor element, and the capacitor element is accommodated in a case made of metal such as aluminum or a case made of a synthetic resin and sealed. Note that aluminum as well as tantalum, niobium, titanium and the like are used as an anode material, and the same type of metal as the anode material is used as a cathode material.
Moreover, as a solid electrolyte used for the solid electrolytic capacitor, manganese dioxide and a 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex are known. In recent years, a technology paying attention to a conductive polymer such as polyethylenedioxythiophene (hereinafter referred to as PEDOT) which has a gentle reaction speed and excellent adhesion between an anode electrode and an oxide coating layer is present (see JP H02-15611 A).
A solid electrolytic capacitor of a type in which a solid electrolyte layer composed of a conductive polymer such as PEDOT is formed in such a winding-type capacitor element is fabricated as follows. First, a surface of the anode foil made of a valve-action metal such as aluminum is roughened by electrochemical etching processing in an aqueous chloride solution so as to form a large number of etching pits. Then, a voltage is applied in an aqueous solution such as ammonium borate so as to form an oxide coating layer which is as a dielectric (chemical conversion). Similarly to the anode foil, the cathode foil is also made of a valve-action metal such as aluminum, but only etching processing is applied to the surface thereof.
The anode foil on which the oxide coating layer is formed on the surface and the cathode foil in which only the etching pits are formed are wound with a separator interposed therebetween so as to form a capacitor element. Subsequently, a polymerizable monomer such as 3,4-ethylenedioxythiophene (hereinafter referred to as EDOT) and an oxidizer solution are discharged to the capacitor element after restoration and formation, respectively, or the capacitor element is immersed in a mixed solution of the both so as to promote polymerization reaction in the capacitor element and generate the solid electrolyte layer composed of a conductive polymer such as PEDOT. After that, this capacitor element is accommodated in an exterior case having a bottom and cylindrical shape so as to fabricate the solid electrolytic capacitor.
the above-described solid electrolytic capacitor has been recently used as in-vehicle equipment or for a general power supply circuit and high withstand voltages of approximately 35V or 63V has been in demand. In order to use the capacitor in such applications, a solid electrolytic capacitor satisfying required items such as heat stability at a high temperature, charging/discharging performances at a low temperature, further ESR reduction and the like is in demand.
Moreover, a lead-free solder having a high melting point has been used recently in view of an environmental problem, and a solder reflow temperature has been further rising from 200 to 220° C. to 230 to 270° C. If reflow-soldering at such a high temperature is performed, a withstand voltage is lowered probably due to heat deterioration or crystallization of the electrolyte layer. Note that, such a problem has also occurred not only in use of EDOT as a polymerizable monomer but in use of other thiophene derivatives, pyrrole, aniline and the like.
The present invention was proposed in order to solve the above-described problems and its object is to provide an electrolytic capacitor having a long life at a high temperature with reduced ESR while charging/discharging performances at a low temperature is ensured, and a manufacturing method thereof.
Moreover, another object of the present invention is to provide an electrolytic capacitor with high withstand voltage capable of preventing deterioration of withstand voltage characteristics caused by lead-free reflow or the like, and a manufacturing method thereof.